The major symptoms of dementia are Alzheimer's disease and mild cognitive impairment, and the patient number is drastically increasing with the advent of aging society. The sole therapeutic drugs therefor are symptomatic improvement drugs such as acetylcholinesterase inhibitors and the like, and the development of a drug capable of halting or delaying the progression of pathology, or a drug having a prophylactic effect has been desired.
The etiology of Alzheimer's disease is considered to be senile plaque formed by accumulation of a peptide consisting of about 40 amino acids, which is called amyloid β (hereinafter sometimes to be simply referred to as Aβ), or nerve cell death. Aβ is a peptide produced by processing a single transmembrane protein amyloid precursor (hereinafter sometimes to be simply referred to as APP), which is a precursor protein, with a degrading enzyme called secretase, and the main molecule species are Aβ40 consisting of 40 amino acids and Aβ42 consisting of 42 amino acids. Of these, Aβ42 aggregates easily and is considered to play a key role in senile plaque formation or nerve cell death (non-patent document 1).
On the other hand, secretase, which is an excision enzyme, is known to include β-secretase for cutting out amino terminal and γ-secretase for cutting out carboxy terminal. γ-Secretase is constituted with presenilin (PS) and 3 kinds of cofactor proteins (nicastrin: NCT, APH-1, PEN-2) etc. (non-patent document 2). A radical treatment drug for Alzheimer's disease, which is based on inhibition of these secretases and suppression of production or secretion of Aβ, has been investigated (non-patent document 1). In the meantime, since γ-secretase is involved not only in the processing of APP but also functions such as activation of Notch receptor playing an important role in cell differentiation by intramembranous cleavage and the like, the development of a drug capable of specifically inhibiting Aβ production alone without influencing other than APP processing has been desired (non-patent document 3).
Patent document 1 describes, as an amyloid β production inhibitor, a compound represented by the following formula:
wherein each symbol is as defined in patent document 1, and the following compound.

Patent document 2 describes, as a compound having an amyloid β level regulating action (use: neurodegenerative disease), a compound represented by the following formula:(A)-LA-(B)-LB-(C)-LC-(D)wherein each symbol is as defined in patent document 2.
Patent document 3 describes, as a cinnamide compound (use: neurodegenerative diseases caused by amyloid β, such as Alzheimer's disease, Down's disease and the like), a compound represented by the following formula:
wherein each symbol is as defined in patent document 3.
Patent document 4 describes, as a polycyclic cinnamide compound (use: neurodegenerative diseases caused by amyloid β, such as Alzheimer's disease, Down's disease and the like), a compound represented by the following formula:
wherein each symbol is as defined in patent document 4, and the following compound.

Patent document 5 describes, as an imidazolyl-phenyl-vinyl-heterocycle derivative (use: Alzheimer's disease), a compound represented by the following formula:
wherein each symbol is as defined in patent document 5.
Patent document 6 describes, as a γ-secretase modulator, a compound represented by the following formula:
wherein each symbol is as defined in patent document 6, and the following compounds.

Patent document 7 describes, as an amyloid β production inhibitor, a compound represented by the following formula:
-D- shows the following structures.
wherein each symbol is as defined in patent document 7, and the following compounds.

Patent document 8 describes, as various heterocyclic compounds, a tachykinin receptor antagonist represented by the following formula:
wherein each symbol is as defined in patent document 8.
Patent document 9 describes a mGluR5 regulator represented by the following formula:
wherein each symbol is as defined in patent document 9.
Patent document 10 describes a 11β-hydroxysteroid dehydrogenase type 1 inhibitor represented by the following formula:
wherein each symbol is as defined in patent document 10.
Patent document 11 describes, as an amyloid β production inhibitor, a compound represented by the following formula:
wherein each symbol is as defined in patent document 11, and the following compounds.

Patent document 12 describes, as an amyloid β production inhibitor, a compound represented by the following formula:
wherein each symbol is as defined in patent document 12, and the following compound.

Patent document 13 describes, as a compound having an amyloid β production inhibitory action, a compound represented by the following formula:
wherein each symbol is as defined in patent document 13.
Patent document 14 describes, as a γ-secretase modulator, a compound represented by the following formula:
wherein each symbol is as defined in patent document 14.
Patent document 15 describes, as a γ-secretase modulator, a compound represented by the following formula:
wherein each symbol is as defined in patent document 15, and the following compound.

Patent document 16 describes, as a γ-secretase modulator, a compound represented by the following formula:
wherein each symbol is as defined in patent document 16, and the following compound.

Patent document 17 describes, as an amyloid β production inhibitor, a compound represented by the following formula:
wherein each symbol is as defined in patent document 17, and the following compound.

Patent document 18 describes, as a γ-secretase modulator, a compound represented by the following formula:
wherein each symbol is as defined in patent document 18.
Patent document 19 describes, as a γ-secretase modulator, a compound represented by the following formula:
wherein each symbol is as defined in patent document 19.